This invention relates generally to a rail fastener of the type used to secure railroad track to an underlying support structure. More particularly, it relates to a rail fastener which provides improved vibration and sound attenuation between the rail and its supporting structure, good resistance to lateral shear forces acting on the rail and good wearing qualities.
As described in detail in U.S. Pat. No. 4,047,663, direct fixation of a rail to a support structure is not a simple matter. Structural integrity must be maintained between the rail and the support structure while vibrations generated in the rail must be attenuated before reaching the support structure. At the same time, the rail fastener must be capable of permitting lateral adjustment or positioning of the rail with respect to the support structure while also providing sufficient resistance to lateral shear forces that can be imposed on the rail especially on curves.
As a vehicle moves along a rail, a wave is built up in the rail in front of the vehicle because of the localized vertical forces applied to the rail by the wheels of the vehicle. The rail acts as a lever causing each portion of the rail to be subjected first to an upward force as the vehicle approaches, a downward force as the wheels roll thereover and another upward force as the vehicle leaves. Where the rail is directly affixed to the support structure, this wavelike motion will produce a pounding action between the rail and the supporting structure. Unless some device is provided between the rail and the structure to absorb this impact, this pounding will ultimately result in failure of the rail, the fastener, or the underlying structure.
In addition to the deleterious effects on the structure caused by the pounding action, undesirable sonic vibrations will be applied to the surrounding structures. Thus, suitable apparatus must be incorporated into the rail fastener device to attenuate the noise which would otherwise be transmitted into surrounding buildings and other structures.
As will be apparent, these competing requirements inevitably lead to compromises. The rail fastener must be both sufficiently rigid to provide structural integrity between the rail and the support structure, and sufficiently non-rigid to attenuate the vibrations transmitted from the rail to the support structure. In an effort to satisfy these and other requirements, rail fasteners have been devised such as those shown in U.S. Pat. Nos. 3,858,804 and 4,047,663 that include a shear pad which is formed of a pair of metallic plates having a layer of elastomeric material sandwiched therebetween. The shear pad is secured to the support structure by a pair of studs or bolts and additional means are provided for laterally positioning the rail with respect to the shear pad and support structure.
These prior art devices tend to be quite complicated. In addition, the anchor between the rail plate and the support structure is a potential source of structural failure. The anchor bolts pull the fastener and the support structure together, thereby placing a portion of the support structure in tension. Such tensioning of the support structure around the anchor bolts contributes to its ultimate fatigue. The pounding of the rail plate against the anchor bolt causes vibration in the anchor bolt which will eventually weaken the portion of the support structure in which it is seated. The pounding may also fatigue the anchor bolt itself. As a result either the anchor bolt will ultimately fracture or the support structure will lose its grip on the bolt.
Since the rail is subject to overturning movements and lateral shear forces, especially on curved portions of the track, it can not be permitted to move laterally when such shear forces are imposed lest the gauge of the track be altered. At the same time, the fastener must be capable of some lateral adjustment of the rail with respect to the support structure so as to maintain the correct gauge as the rail is worn down through use.
In the above referenced '804 patent, the rail can be adjusted by adjusting the position of rail support clip 50 or the rail fastener itself. To lock the support clip or rail fastener in position, serrated edges are provided on the clip or the fastener and on mating elements which engage these elements. As will be apparent, the use of serrated components increases the cost of the device and greatly limits the number of adjustments that can be made to its position. The '663 patent discloses the use of eccentric cams 44, 46 to provide lateral adjustment of the rail plate. This arrangement, however, is likely to require extensive inspection and maintenance to ensure that the cams remain in the desired position and that the gage remains constant. In addition, while the Pandrol clips disclosed in the '663 patent provide a resilient restraint on upward movement of the rail under the leverage action of a vehicle moving along the rail, it will be apparent that the fastener shown in the '663 patent is complicated in design and manufacture and that its installation and maintenance is likely to be expensive.